The present invention relates to navigation reference systems and more particularly concerns establishment and operation of multi-unit underwater reference systems.
In many types of operations on and below the surface of the sea, establishment of position is required. Among the various types of operations requiring such undersea position reference, are operations involving undersea construction, exploration and installation, such as carried out in the searching for and recovery of undersea minerals of various types. Underwater navigation systems may provide a common relative position reference to allow precise "reconstruction" of naval exercises, position histories of devices used in underwater acoustic or oceanographic research or other such applications, either underwater or in the atmosphere, where it is desired to accurately determine the position of one or more objects. Other examples of uses of such a reference system include submarine position fixing, surface vessel position fixing, automatic generation of track plots of such units during conduct of training and/or tactical exercises and generation of data for determination of drift tracks of devices such as "drift bottles," used to map ocean currents both at and below the surface. In the course of some of these and other operations, it is often necessary to establish and monitor the position of self-propelled or towed underwater vehicles or objects within a selected geographic area. Various types of such operations and systems heretofore available for establishing such positions are described in a paper entitled Underwater Arctic Navigation presented by J. A. Cestone and E. St. George, Jr., to the Royal Institute of Navigation, Feb. 27, 1974. Pages 36-38 of this paper describe various types of ship maneuvers employed in locating such reference transponders. U.S. Pat. No. 3,860,900 for a Method of Monitoring the Position of Towed Underwater Apparatus describes similar methods of location.
In the system described in U.S. Pat. No. 3,860,900, the three dimensional positions of a number of underwater transponders are fixed by an involved procedure often requiring long periods of operation of a surface ship. Thus, in order to determine depth of the transponder according to the method of U.S. Pat. No. 3,860,900, a surface ship is maneuvered in a cloverleaf pattern above each transponder, continuously interrogating the transponder until a minimum round trip acoustic travel time is determined to thus determine transponder depth. Thereafter, the ship is maneuvered to cross each base line, established between each pair of transponders, at its approximate midpoint and at right angles, again interrogating transponders to determine a minimum round trip acoustic travel time. This procedure is time consuming, requires the use of a surface ship for a considerable period of time and is either inaccurate or requires difficult and time consuming velocity measurement corrections. Such velocity measurement corrections are necessary because the distance measurements depend upon the velocity of sound in the water which is used together with measured time intervals to calculate distance. Sound velocity varies with many water parameters including depth, salinity and temperature. Consequently for accuracy of measurements, these variable parameters must be repetitively determined to obtain sufficiently precise corrections for the distance measurements based upon sound velocity. These corrections are of more significance because of the fact that measurements are made between the transponder at the sea bottom and a receiver at the surface. Thus, the sound being measured must travel between the bottom and the surface along a path that is subject to significant variations in pressure, temperature and salinity.
In the system of the U.S. Pat. No. 3,860,900, after the position of the transponders has been fixed, one must periodically fix the position of a surface vessel relative to the transponders by interrogating the transponders and thereafter periodically fix the position of the unknown towed equipment by employing a remote interrogation transducer which is commanded by the surface ship and then caused to interrogate the transponders. Thus, after establishment of the position of the transponders, the use of a surface ship for periodically and repetitively commanding interrogation is still required and even the remote interrogation transducer is required to actively transmit to the responders.
For those underwater reference systems which may be employed for navigation without the use of a command interrogation surface ship as in U.S. Pat. No. 3,860,900, a precise and relatively driftfree time synchronization is required. In such a system, each of a plurality of underwater transponders may transmit signals at precisely known absolute or relative times for reception by a receiving station that is to locate its own position with respect to the transmitting underwater reference stations. Thus, presuming each underwater reference station transmits at exactly the same time, the receiving station may utilize conventional hyperbolic navigation techniques to determine its position with respect to the transmitting stations. In such conventional navigation techniques, as is well known, the time difference between receipt of signals at a receiving station from two simultaneously transmitting stations of known position will establish a first curve (an hyperbola) passing through the receiving station and uniquely positioned with respect to the transmitting pair. Receipt of similar signals from a second pair establishes a second curve which intersects the first curve to establish the receiving position. However, for such systems, one must first establish the position of the underwater transmitting stations and then, having located these, must ensure that the transmissions therefrom are precisely synchronized so that such stations either transmit simultaneously or at precisely fixed times with respect to one another. Such timing requires equipment of high precision and great cost. For equipment to maintain accuracy within a tenth of a millisecond for as little as ten days, exceedingly expensive system clocks may be required. Thus, such underwater position reference systems are either expensive, have very shot lives, or require frequent resetting or checking from another station such as a surface ship or the like.
Accordingly, it is an object of the present invention to provide methods and apparatus for underwater navigation that eliminate or minimize the above-mentioned disadvantages.